This invention relates to a semiconductor device and more specifically to an electrode on the semiconductor chip for connecting a semiconductor element thereon with an outside lead by means of a thin bonding wire.
A semiconductor device comprising a semiconductor chip and lead frame is well known. A semiconductor chip having one or more semiconductor elements such as diodes or transistors is mounted on a mounting area of a lead frame, and each region of the element such as the emitter, base or collector is connected with a lead of the lead frame by a thin bonding wire of gold or aluminum. For connection with the bonding wire each region of the semiconductor device is provided with an electrode. In an integrated circuit chip, regions are usually connected to each other by interconnection on the chip and, for connection with a lead outside, a special part of the interconnection called a bond pad or electrode is prepared. The bonding wire is connected with the electrode or the bond pad by means of thermocompression or thermosonic bonding. In carrying out this bonding, a small ball is formed at the end of a bonding wire by passing a H.sub.2 torch over the end of the wire or by capacitance discharge. Then the ball is pressed onto the electrode under high temperature while the wire is excited ultrasonically.
As material of the bonding wire, copper is preferred rather than gold or aluminum, since gold is expensive and with aluminum it is difficult to make the ball, thus requiring a wide area of the substrate.
In the course of developing copper wire application to a semiconductor chip with conventional aluminum electrodes, however, two kinds of failure have been experienced. One type of failure is disconnection of the copper ball from the aluminum layer of the electrode due to insufficient pressure or temperature. The other type of failure is the destruction of the semiconductor substrate under the electrode. Bonding depends upon various conditions--mechanical pressure, temperature, strength of the ultrasonic wave, time, etc. As copper is rather hard, to make the ball at the end of the wire flat for bonding requires much pressure, and this frequently brings about the destruction of the substrate. Thus connecting a copper wire to an aluminum electrode can only be accomplished under very limited conditions, and small fluctuations of pressure, temperature or such often lead to detrimental failure.
It is the object of the present invention to provide a semiconductor device having a reliable connection with a lead outside the chip. It is another object of the invention to provide an electrode of a semiconductor chip which is suitable and easy for bonding with copper wire.
It is still another object of the invention to provide a method of manufacturing such electrodes.